This is another useful presentation from Autodesk Building Solutions for Revit users. One can learn how to position and revise couplers and caps to rebar and rebar sets. This is one of the useful features of Revit 2017.
Rebar Couplers are useful to detail your reinforcement drawing and document. Rebar couplers attach adjoining rebar and at the same time make the design of reinforced concrete simple.
How to access Rebar Couplers:
Structure tab Reinforcement panel (Rebar Coupler)
Modify | Structural Rebar tab Rebar Coupler panel (Insert Coupler)